1. Field of the Invention
The present invention relates to a display apparatus including a light emitting device, and more particularly to a layout thereof.
2. Description of the Related Art
Organic EL panels using organic EL elements are conventionally known, and the development of such organic EL panels has been advancing. In organic EL panels, the organic EL elements are arranged in a matrix, and the light emission of each of the organic EL elements is individually controlled to perform display. In particular, in an active matrix type organic EL panel, each pixel includes a TFT for display control, and the light emission of each pixel can be controlled by the operation control of the TFT. Consequently, it is possible for the active matrix type organic EL panel to perform display with a very high accuracy.
FIG. 12 shows an example of a pixel circuit in the active matrix type organic EL panel. A data line DL, to which a data voltage expressing the luminance of a pixel is supplied, is connected to the gate of a drive TFT 12 through an n-channel selection TFT 10, the gate of which is connected to a gate line GL. Moreover, an end of a holding capacitor 14, the other end of which is connected to a capacitance line SC, is connected to the gate of the drive TFT 12 to maintain the gate voltage of the drive TFT 12.
The source of the drive TFT 12 is connected to an EL power supply line, and the drain thereof is connected to the anode of an organic EL element 16, the cathode of which is connected to a cathode power supply.
Such pixel circuits are arranged in a matrix, and a gate line provided to each horizontal line turns to an H level at predetermined timing to turn on the selection TFT's 10 on the line. In this state, because data voltages are sequentially supplied to the data lines, the data voltages are held by the holding capacities 14, and the data voltages at that time are held even if the gate line is turned to an L level.
The drive TFT's 12 operate according to the voltages held by the holding capacitors 14, and corresponding drive currents flow from the EL power supply to the cathode power supply through the organic EL elements 16. Thus, the organic EL elements 16 emit light according to the data voltages.
The gate lines are sequentially turned to the H level to supply input video signals to corresponding pixels as data voltages. As a result, the organic EL elements 16 arranged in the matrix emit light according to the data voltages to perform the display of the video signals.
Such pixel circuits have a problem in that display quality is lowered owing to the dispersion of luminance at the time of the dispersion of the threshold voltages of the drive TFT's of the pixel circuits arranged in the matrix. It is difficult to unify the characteristics of the TFT's constituting the pixel circuits of the whole display panel, and it is difficult to prevent the dispersion of the threshold values of the turning on and off of the TFT's.
Accordingly, for example, Published Japanese Translation of PCT Application No. 2002-514320, Japanese Patent Laid-Open Publication No. 2005-128521 and the like have proposed circuits for preventing the influence of the variations of the threshold values of TFT's.
However, these proposals need two or more control lines for the control of each pixel circuit. That is, the above-mentioned circuit shown in FIG. 12 needs only the gate line GL as the other control lines besides the data line DL and the power supply line, which extend in the vertical direction, to each pixel, but each of the pixel circuits disclosed in the above documents needs at least two control lines in addition to the gate line.
Consequently, these proposals have a problem in that not only the control lines but also connection lines to connect the control lines with transistors increase, which decreases the aperture ratio of the display apparatus.
Accordingly, it is desired to keep the aperture ratios at comparatively high values by arranging the wiring and the like efficiently.